In our British patent specification No. 1365876, we described a security paper having a porous tape incorporated therein as a security device. The provision of a security device in the form of a tape or strip is conventional. The tape or strip is normally incorporated in the paper as the paper is made on a cylinder mould papermaking machine. In such a machine paper is deposited on a mesh which rotates in a paper furnish whilst liquid flows from the furnish into the mesh. The tape or strip usually incorporated into paper as a security device is normally very narrow, e.g. 0.5 mm in width. This narrow width is necessary because the tape or strip is of impermeable material, e.g. polyester plastics film and therefore blocks the flow of liquid through the mesh at the point where it lies. If the tape or strip is too wide, paper fibres cannot bridge the tape or strip and the tape is therefore exposed on one side of the paper instead of being incorporated into the thickness of the paper. The tape or strip disclosed in our specification No. 1365876 was porous and therefore could be incorporated into paper on a cylinder mould-making machine even when of substantial width, e.g. 11/2" wide since it did not block the flow of liquid from the furnish through the wire mesh of the cylinder mould machine.